With the recent wide spread use of portable devices, such as mobile phones and notebook personal computers, there has been a strong demand to develop small, light secondary batteries having high energy density. Among secondary batteries satisfying such a demand are lithium-ion secondary batteries using lithium, lithium alloy, metal oxide, or carbon as a negative electrode active material. Such lithium-ion secondary batteries are being actively researched and developed.
A lithium-ion secondary battery that uses, as a positive electrode active material, lithium composite oxide, particularly, lithium-cobalt composite oxide (LiCoO2), which is relatively easily synthesized, supplies a 4 V-level high voltage. For this reason, such lithium-ion secondary batteries are being commercialized as batteries having high energy density. To obtain excellent initial capacity characteristics or cycle characteristics, there have been developed many batteries using lithium-cobalt composite oxide. Various fruits have already been produced.
However, lithium-cobalt composite oxide (LiCoO2) uses a cobalt compound, which is rare and expensive, as a raw-material and therefore causes a cost increase. For this reason, there is a demand for a cheaper alternative serving as a positive electrode active material. Reducing the cost of a positive electrode active material and thus producing a cheaper lithium-ion secondary battery is of great industrial significance, since it can contribute to the downsizing and weight-reduction of portable devices which are being currently widely used.
Among new positive electrode active materials for lithium-ion secondary batteries are lithium-manganese composite oxide (LiMn2O4) using manganese, which is cheaper than cobalt, and lithium-nickel composite oxide (LiNiO2) using nickel.
Lithium-manganese composite oxide is formed of cheap materials and has excellent thermal stability. Accordingly, it can be said to be a promising alternative material to lithium-cobalt composite oxide. However, lithium-manganese composite oxide has difficulty in meeting a demand to increase the capacity of lithium-ion secondary batteries, which has been raised year by year, since its theoretical capacity is only about half that of LiCoO2. As for lithium-nickel composite oxide, it has lower cycle characteristics than lithium-cobalt composite oxide and is more likely to lose battery performance when used or stored in a high-temperature environment.
On the other hand, lithium-nickel-manganese composite oxide has thermal stability and durability similar to those of lithium-cobalt composite oxide and is a promising alternative to lithium-cobalt composite oxide. For example, Patent Literature 1 proposes, as a precursor of a positive electrode active material containing lithium-manganese-nickel composite oxide, manganese-nickel composite hydroxide particles having a manganese-nickel ratio of substantially 1:1, an average particle diameter of 5 to 15 μm, a tap density of 0.6 to 1.4 g/mL, a bulk density of 0.4 to 1.0 g/mL, a specific surface area of 20 to 55 m2/g, and a sulfate group content of 0.25 to 0.45% by mass. Patent Literature 1 also discloses, as a production method of the manganese-nickel composite hydroxide particles, causing a mixed aqueous solution of a manganese salt and a nickel salt having a manganese-nickel atomic ratio of substantially 1:1 to react with an alkali solution in an aqueous solution having a pH of 9 to 13 in the presence of a complexing agent on an appropriate stirring condition while controlling the degree of oxidation of manganese ions to a predetermined range and then coprecipitating the resulting particles.
There has been also proposed a positive electrode active material whose particle structure is controlled so as to improve cycle characteristics or output characteristics. For example, Patent Literature 2 discloses nickel composite hydroxide consisting of approximately spherical secondary particles which are agglomerations of multiple primary particles and which have an average particle diameter of more than 7 μm and equal to or less than 15 μm. The nickel composite hydroxide has [(d90−d10)/average particle diameter] of 0.55 or less, and [(d90−d10)/average particle diameter] is an index indicating the extent of the particle size distribution. Patent Literature 2 also discloses a positive electrode active material for nonaqueous electrolyte secondary batteries obtained using this nickel composite hydroxide. The positive electrode active material has an average particle diameter more than 8 μm and equal to or less than 16 μm, and [(d90−d10)/average particle diameter], which is an index indicating the extent of the particle size distribution, is 0.60 or less. The positive electrode active material includes a shell and a hollow present inside the shell.